The present invention relates to a process for producing acyloxysilanes, and more particularly, to an improved process for the continuous production of acyloxysilanes in a system which substantially increases capacity over prior art systems.
Acyloxysilanes are well known cross-linking agents for 1-part room temperature vulcanizable silicone rubber compositions. One common acyloxysilane cross-linking agent is methyltriacetoxysilane.
Acyloxysilane cross-linking agent has been made by the reaction of an appropriate chlorosilane with a carboxylic anhydride or with a carboxylic acid. One process for preparing acyloxysilanes by reacting a chlorosilane with a carboxylic acid or carboxylic acid anhydride in the presence of an iron complexing agent is disclosed in U.S. Pat. No. 3,974,198. In U.S. Pat. No. 3,974,198, an aliphatic carboxylic acid, such as, glacial acetic acid, was added at the top of a distilling column to a refluxing mixture of a chlorosilane in an organic solvent, such as hexane, containing an iron complexing agent. After the completion of the addition of the aliphatic carboxylic acid, the solvent was removed by distillation, and the acyloxysilane compound was eventually isolated from the mixture.
Aliphatic carboxylic acid in the vapor phase is passed upwards from the bottom of a column filled with Raschig rings countercurrent to a flow of chlorosilane in U.S. Pat. No. 4,176,130. However, in U.S. Pat. No. 4,176,130 only a limited amount of aliphatic carboxylic acid is introduced into the column so that the feed rate of the carboxylic acid does not exceed 1.3 moles per gram atom of silicon-bonded chlorine in the column. Thus, at most, only a very slight excess of aliphatic carboxylic acid is introduced into the column in U.S. Pat. No. 4,176,130. The liquid glacial acetic acid vaporizes as it enters the column, and the acetic acid vapor rises against the flow of the chlorosilane. In more preferred embodiments, U.S. Pat. No. 4,176,130 specifies operation of the column at 50 to 300 mm (Hg) absolute such that the product exiting from the reboiler is substantially free of acetic acid. The reduction of absolute pressure reduces the temperatures in the column, reduces the density of the vapor phase, and reduces the equilibrium concentration of the chlorosilane monomer (methyltrichlorosilane or ethyltrichlorosilane) in the liquid phase (primarily liquid acetic acid). Thus, operation at pressures substantially below atmospheric pressure reduces the rate of reaction due to lower temperature and lower concentration of chlorosilane reactant in the liquid phase in the column and reduces the rate of mass transfer of unreacted chlorosilane from the vapor phase to the liquid phase. In addition, the slight excess of carboxylic acid results in a lower reaction rate in contrast to an excess of about 30 to 100% above stoichiometric requirements. The practical use of acyloxysilanes produced by this method requires that the remaining unreacted chlorine attached to silicon be reduced to a level of at least 50 ppm. and preferably below 10 ppm. The process described in U.S. Pat. No. 4,176,130 reduces the rate of reaction to a level that seriously limits the column throughput if the product requirement of less than 50 ppm., and preferably less than 10 ppm. of residual chloride, is to be obtained. The formation of dimer at the bottom of the column is a significant side reaction that results from thermal decomposition of monomeric acyloxysilanes or from a reaction of the chlorosilane with acyloxysilane or both. The dimer is represented by the following formula: EQU R.sub.n (R'COO).sub.3-n Si--O--Si(R'COO).sub.3-n R
wherein R and R' are alkyl radicals generally of 1 to about 8 carbon atoms and n is 1 to 3. When the reactants form dimer, it reduces the amount of product. Thus, it is desirable to reduce the amount of dimer formed from the reaction, or to increase column throughput to provide commercial quantities of the acyloxysilanes even though the dimer is formed.
The continuous prior art processes are also disadvantageous insofar as low boiling contaminants generally accumulate in the column, and the low boiling contaminants reduce the column temperature. Accordingly, it is desirable to provide a process which eliminates this problem so that the column temperature can be maintained at a steady maximum temperature.